1. Field of the Invention
The present invention relates to a voltage drop amount measurement circuit, and particularly to a voltage drop amount measurement circuit for measuring that the amount of drop in a power supply voltage exceeds a predetermined value.
2. Description of Related Art
In recent years, a power supply voltage of a semiconductor device is coming to decrease. In a semiconductor device, a power supply voltage drops according to the amount of current consumed in an operation. The semiconductor device having a low power supply voltage has less margin for the power supply voltage of a circuit. In such semiconductor device, there is a problem that a drop of the power supply voltage could cause the circuit operation to be unstable.
Thus the amount of drop of the operating power supply voltage in the semiconductor device has been measured. An example of a technique concerning the measurement of the drop amount of power supply voltage is disclosed in Japanese Unexamined Patent Application Publication No. 2004-146612. A voltage drop amount measurement circuit 100 disclosed in Japanese Unexamined Patent Application Publication No. 2004-146612 is shown in FIG. 9. As shown in FIG. 9, the voltage drop amount measurement circuit 100 includes oscillator circuits 101A to 101E, a decoder 102 and selector 103.
The oscillator circuits 101A to 101E are a ring oscillator that changes an oscillating frequency according to a power supply voltage. The decoder 102 outputs an operation instruction to one of the oscillator circuits 101A to 101E. The selector 103 selects the oscillator circuits operating according to the operation instruction from the decoder 102 and outputs an output from the selected oscillator circuit as an output signal SO. Here, the relationship between the oscillating frequency and the amount of drop in the power supply voltage is recorded on a table or the like and by comparing the oscillation frequency of the output signal SO using this table, the amount of drop of the power supply voltage around the operating oscillator circuit can be measured.
The oscillator circuits 101A to 101E of the output signal SO are placed at different positions of the semiconductor device. Therefore, by changing the oscillator circuit to operate, spread of the amount of drop in the power supply voltage for the semiconductor device can be found out.
However in this technique, the amount of power supply voltage drop is measured using an oscillating frequency of a ring oscillator. As the ring oscillator is made up of a plurality of inverters connected to form a circle, the circuit size becomes comparatively large. Thus by placing many oscillator circuits in a semiconductor device as with the voltage drop amount measurement circuit 100, there is a problem that the chip size increases.
Further, the ring oscillator requires predetermined time until the oscillating frequency is stabled. Accordingly it has been difficult to measure fluctuations in power supply voltage that are generated in a period of about 1 cycle of a clock. As the semiconductor device generally operates according to a clock, a voltage drop even in 1 cycle of a clock could be a problem. However, in the voltage drop measurement circuit of this technique, it is difficult to measure a drop in the power supply voltage in such a short period.